-LRB- CNN -RRB- -- Clusters of Roman skulls have been discovered deep below London 's Liverpool Street by construction workers digging a new rail route through England 's capital .

Tunnelers working on the Crossrail project found about 20 skulls , deep beneath the 16th century Bedlam burial ground in the center of the city , Crossrail said in a statement .

Read more : Rail excavation unearths suspected ` plague pit '

The human skulls -- as well as fragments of Roman pottery -- were found in sediment of the historic river channel of the River Walbrook , a tributary of the River Thames , it said .

Roman-era skulls had been found along the path of the River Thames throughout London 's history , fueling speculation they were the heads of the victims of rebels fighting under Queen Boudicca against Roman occupation , lead archaeologist Jay Carver said .

Read more : London dig turns up slice of Roman life

`` We now think the skulls are possibly from a known Roman burial ground about 50 meters up river from our Liverpool Street station worksite , '' he said . `` Their location in the Roman layer indicates they were possibly washed down river during the Roman period . ''

Crossrail said tunnelers at Liverpool Street also discovered medieval wooden structures , which archaeologists believe may have formed part of the Bedlam cemetery walls .

About 3,000 skeletons previously discovered at the Bedlam burial ground are set to be relocated next year .

The Museum of London Archaeology will analyse the finds , which are the latest in more than 10,000 archaeological items found across 40 project construction sites , Crossrail said .

Read more : Body found under parking lot is King Richard III

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Construction workers digging a tunnel in London have found about 20 Roman-era skulls

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The workers were digging a tunnel for the Crossrail rail project spanning the city

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Archaeologists believe the skulls were washed down stream from an old burial ground

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More than 10,000 archaeological items have been found since the project began in 2009